In wellbore drilling situations that use a drilling rig, a drilling fluid circulation system circulates (or pumps) drilling fluid (for example, drilling mud) with one or more mud pumps. As illustrated in prior art FIGS. 1 and 2, the drilling fluid circulation system 10 moves drilling mud (fluid, F) down into the wellbore 50 through special pipe referred to in the art as drill pipe 12, and drill collars which are connected to the drill sting. The fluid exits through ports (jets) in the drill bit, picking up cuttings C and carrying the cuttings up the annulus 40 of the wellbore 50 (FIG. 2). Referring again to FIG. 1, the mud pump 30 takes suction from mud tanks 22 and pumps mud F out discharge piping 24, up the stand pipe 26, through rotary hoses 28, through Kelly or top drive unit 31, and into a central bore of the drill pipe 12, drill collars, and bit. Mud F and cuttings C return to the surface up annulus 40 (FIG. 2). At the surface, the mud and cuttings leave the wellbore through an outlet (not shown), and are sent to a cuttings removal system via mud return line 60. At the end of the return lines, mud F and cutting C are flowed onto a vibrating screen known in the art as a Shale Shaker 62. Finer solids may be removed by a sand trap 64. The mud may be treated with chemicals stored in a chemical tank 66 and then provided into the mud tank 22, where the process is repeated.
The drilling fluid circulation system delivers large volumes of mud flow under pressure for drilling rig operations. The circulation system delivers the mud to the drill stem to flow down the string of drill pipe and out through the drill bit appended to the lower end of the drill stem. In addition to cooling the drill bit, the mud hydraulically washes away the face of the wellbore through a set of openings in the drill bit. The mud additionally washes away debris, rock chips, and cuttings, which are generated as the drill bit advances. The circulation system flows the mud in an annular space on the outside of the drill stem and on the interior of the open hole formed by the drilling process. In this manner, the circulation system flows the mud through the drill bit and out of the wellbore.
The mud flows through the drill stem and the annular space at a sufficient velocity to move debris, chips and cuttings, which are heavier than the mud, to the surface. The velocity of the mud should also be sufficient to cool the drill bit. The wellhead pressures at the pump are sufficiently high to flow the mud at the desired velocity and also to overcome substantial flow pressure resistance along the flow path. In some situations, the circulation system can flow the mud through the drill bit and the wellbore at high volumetric flow rates (for example, 500 to 1,000 gallons per minute) and at pressures as high as 5,000 PSI. The circumstances under which the drilling rig is used can cause the drill string to experience a pack off, for example, when the cuttings or debris (or both) clog the flow path of the drilling mud in the annulus between the borehole and the drill string, drill collars, or drill bit, or combinations of them. Consequently, a downhole pressure can rapidly increase resulting in fracture of one or more downhole formations and possibly even loss of the wellbore.
Like reference numbers and designations in the various drawings indicate like elements.